1. Field of the Invention
The present invention relates to one component silicone compositions which cure at room temperature to form elastomeric products. More particularly the present invention relates to room temperature vulcanizable (RTV) silicone rubber compositions having high elongation and low modulus.
2. Description of the prior art
Oxime silanes are known in the art. U.S. Pat. No. 3,189,576 describes oxime silanes useful for forming one component room temperature curing coating and caulking compositions. This class of organosilicon intermediates is described by the general formula (X=NO).sub.a SiR'.sub.4-a. Although many publications describe the use of oxime silanes as crosslinkers in RTV silicone compositions, none discloses the specific use or advantage of employing a phenyl substituted tris-functional ketoxime silane. Tris-functional describes silanes containing three hydrolyzable groups bonded to the silicon atom.
U.S. Pat. No. 3,996,184 teaches the use of a difunctional amide, specifically methyl vinyl di-(N-organo acetamido)silane, as a coupler and aminoxy functional polysiloxane as crosslinker to achieve a low modulus sealant. Such aminoxy functional polysiloxanes are also described in U.S. Pat. Nos. 3,341,486; 3,441,583; 3,528,941; and 3,926,199. A disadvantage of such systems is the limited availability of difunctional amides and also the undesirable use of solvents employed therewith. Furthermore, the methyl vinyl di-(N-organo acetamido)silane lacks adequate color stability since it darkens over a relatively short storage time.
It is desirable to find a one component, high elongation, low modulus sealant with good adhesion to various substrates without the use of such solvents or amides and which have better color stability. The composition of this invention can provide low modulus RTV compositions without the use of solvents and amides and which utilize phenyl substituted tris-functional ketoxime silanes.
U.S. Pat. No. 4,546,017 teaches the use of titanium chelates to achieve low modulus sealants with improved adhesion. Such sealants are difficult to produce in desirable white colors due to the presence of titanium chelates which are used at levels which produce a yellow hue to the sealant. Also, such sealants tend to undesirably produce a separation or migration of the titanium chelate from the main body of the sealant. The compositions of this invention produce low modulus sealants with sufficient adhesion for commercial applications yet with significantly reduced yellowing and no separation of ingredients due to the lack of high levels of titanates. The silanes of this invention also exhibit beneficial characteristics such as shelf life, adhesion, a relatively low corrosive nature, and generally low odor. Methyl tris-(methyl ethyl ketoximino)silane, vinyl tris-(methyl ethyl ketoximino)silane and tetrakis-(methyl ethyl ketoximino)silane are the predominant oxime silanes used commercially and which are most readily available for use as crosslinkers in oxime RTV compounds. Heretofore it has been very difficult to achieve low modulus RTV sealants with these oxime silanes without encountering several disadvantages. Currently, a common way to achieve low modulus oxime RTV silicone compounds is to add very high levels of non-reactive or hydroxyl free silicone plasticizer such as trimethylsiloxy-endblocked polydimethylsiloxanes. Unfortunately, the use of such levels of plasticizer tends to greatly increase the likelihood of plasticizer bleed in the sealant. Plasticizer bleed is a disadvantage because it increases unsightly dirt pick-up in adjacent areas in masonry applications. Another disadvantage is that sealants which bleed may also harden and increase modulus more so than if the plasticizer stayed within the sealant. Additionally, hardening may produce greater stresses at the adhesive interphase, jeopardizing the bond, i.e. adhesion, to the substrate. Also, using high levels of plasticizer often leads to slower tack free times and rubberization times of the sealant. This increases the likelihood of cracking during cure if movement occurs. By using phenyl substituted tris-functional silanes, much lower modulus can be achieved than with using methyl tris-(methyl ethyl ketoximino)silane, vinyl tris-(methyl ethyl ketoximino)silane, tetrakis-(methyl ethyl ketoximino)silane or combinations thereof in similar compositions and while achieving a reasonable cure profile. This can be achieved at the same or lower plasticizer levels in comparable formulations. Using methyl tris-(methyl ethyl ketoximino)silane as the sole crosslinker can produce a low modulus sealant by using very low amounts of tin catalyst. This kind of formulation (because of its long tack free time and rubberization time), however, will have an undesirable cure profile and is more likely to crack during cure, severely limiting usefulness. Methyl tris-(methyl ethyl ketoximino)silane/vinyl tris-(methyl ethyl ketoximino)silane combinations can be employed with low levels of tin catalyst, but the resulting sealant will have lower elongation and higher modulus at 100% elongation than a comparable formulation using phenyl tris-functional ketoximino silanes of this invention. The same is generally true for combinations of tetrakis-(methyl ethyl ketoximino)silane and vinyl tris-(methyl ethyl ketoximino)silane or methyl tris-(methyl ethyl ketoximino)silane. Thus, the RTV silicone compositions of this invention provide a sealant with the same or lower cure profile, including skin time, tack free time and scratch time, but simultaneously have a much higher elongation at break and much lower modulus. The same compositions also have a desirable cure profile in that usually, long skin over times and short scratch times are observed. Long skin over times are desirable when considering tooling and the working time of the sealant in its application. Short scratch times relative to skin over times are desirable in that they tend to provide resistance to cracking during the cure of the sealant due to movement.